Strong enhancement of trichloroethylene degradation in ferrous ion activated persulfate system by promoting ferric and ferrous ion cycles with hydroxylamine

Separation and Purification Technology 147 (2015) 186–193

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Strong enhancement of trichloroethylene degradation in ferrous ion activated persulfate system by promoting ferric and ferrous ion cycles with hydroxylamine Xiaoliang Wu, Xiaogang Gu, Shuguang Lu ⇑, Zhaofu Qiu, Qian Sui, Xueke Zang, Zhouwei Miao, Minhui Xu State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 7 November 2014 Received in revised form 24 March 2015 Accepted 23 April 2015 Available online 1 May 2015 Keywords: Persulfate Ferrous ion Trichloroethylene Hydroxylamine Reactive oxygen species

a b s t r a c t The persulfate (PS) activated by ferrous ion (Fe(II)) system could generate reactive oxygen species capable of degrading refractory organic contaminants trichloroethylene (TCE). Nevertheless, the slow conversion from ferric ion (Fe(III)) back to Fe(II) limits its widespread practical application. Therefore, different reducing agents, i.e., hydroxylamine (HA), sodium thiosulfate, ascorbic acid, sodium ascorbate and sodium sulfite, were added into PS/HA system for accelerating the Fe(II) regeneration, and HA was most efficient in TCE degradation. The effects of HA, Fe(II) and PS concentrations were also evaluated in PS/Fe(II)/HA system. The results indicated that a proper HA and Fe(II) concentrations were needed in practical application, too low or too high dosages were adverse to TCE degradation. Moreover, TCE degradation was increased with the increasing of PS dosage over the tested range. The radical scavenging tests  confirmed that the primary reactive oxygen species were sulfate radicals (SO 4 ), hydroxyl radicals ( OH) 2    ) in PS/Fe(II)/HA process. Both inorganic anions (Cl , HCO , SO , NO ions) and superoxide radical (O 2 3 4 3 and natural organic matter had inhibitory effects on TCE removal, and the suppressive effects of inorganic 2    anions can be ranked in an ascending order of NO3 < SO4 < Cl < HCO3 in PS/Fe(II)/HA system. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The chlorinated solvent trichloroethylene (TCE), one of the most widespread and abundant contaminants in groundwater and soils due to the improper treatment and disposal [1], is widely used as a metal degreaser, a chemical intermediate and extractant, an industrial solvent, and a household cleaner and solvent [2]. It is reported that TCE could affect principally the human central nervous system (CNS), for example, short exposure can result in subjective symptoms such as headache, nausea, and incoordination; and longer exposure may result in CNS depression, hepatorenal failure, and increased cardiac output [3]. Therefore, it is important to develop an effective remediation technology in reducing the potential risk of those toxic contaminants in environment. Owing to the rapid remediation and relatively cost-effective technology, in situ chemical oxidation (ISCO) has been developed as a technique of interest [4]. Several chemical oxidants such as H2O2, permanganate, ozone and persulfate are commonly used in ISCO processes [5]. Among all commonly used ISCO oxidants, persulfate (peroxodisulfate, S2O2 8 , ⇑ Corresponding author. Tel.: +86 21 64250709; fax: +86 21 64252737. E-mail address: [email protected] (S. Lu). http://dx.doi.org/10.1016/j.seppur.2015.04.031 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.

PS, E0 = 2.01 V) attracts wide concern due to its easy storage and transport, high aqueous solubility, high stability, and comparatively low cost [6]. Moreover, PS can be activated by heat [7,8], UV or VUV light irradiation [8,9], alkaline pH [10], transition metals [11–13], iron oxides [14], or zero-valent iron (ZVI) [15] to generate 0 a stronger oxidant sulfate radical (SO 4 , E = 2.5–3.1 V) [16], which is comparable to that of hydroxyl radicals (OH, 2.7 V) [17], but more efficient than OH in degradation of some refractory organic contaminants for its selective oxidation capacity [16]. Although heat and UV are effective methods to activate PS, the intrinsic disadvantage of high cost and complexity limit their widespread practical application. Among the common transition metal ions, ferrous iron (Fe(II)) has been found to be the most suitable activator of PS to produce SO 4 in practical application due to its advantages of environmentally friendly nature, cost effectiveness and high activity [18]. Similar to the classical Fenton’s reaction, PS could be activated by Fe(II) to generate SO 4 and sulfate, as depicted by Eq. (1) [19].  2þ  S2 O2 ! Fe3þ þ SO2 8 þ Fe 4 þ SO4

k ¼ 15:33 M1 s1

ð1Þ

However, PS activated by Fe(II) process has some intrinsic drawbacks, for instance, a rapid accumulation of Fe(III), the slow conversion from Fe(III) to Fe(II), the formation of Fe(III) precipitates

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(i.e. iron oxide and/or hydroxide) at pH > 4, and SO 4 scavenging by excess Fe(II) [20], therefore limit its application unfortunately [20,21]. Many efforts have been employed to alleviate and/or overcome these drawbacks. Previous studies have indicated that gradual addition of Fe(II) could effectively enhance the contaminant degradation efficiency [13,22]. Besides, many organic chelating agents, including citric acid (CA) or citrate [18,23,24], ethylenediaminetetraacetic acid (EDTA) [24,25], sodium triphosphate [18,24], (S,S)-ethylenediamine-N,N-disuccinic acid (EDDS) [18], hydroxylpropyl-b-cyclodextrin (HPCD) [25] and 1-hydroxyethane-1,1diphosphonic acid (HEDPA) [24], were applied in PS/Fe(II) system to maintain the iron solubility and improve the generation of reactive oxygen species. In addition, some heterogeneous iron activators were used to avoid the accumulation of Fe(III) precipitates and enlarge the optimum pH range [26,27]. Furthermore, UV [28] and electrochemistry [29] were successfully employed to accelerate PS/Fe(II) process by regenerating Fe(II). Nevertheless, the major drawback in PS/Fe(II) system was the slow conversion from Fe(III) to Fe(II). For the purpose of promoting the regeneration of Fe(II), we could take into account some reducing agents with low reaction rates to reactive oxygen species generated in PS/Fe(II) system. Hydroxylamine (NH2OH, HA), a well-known reducing agent for reducing Fe (III) to Fe(II), was applied in many applications such as in the determination of Fe(II) concentration with the spectrophotometric method using 1,10-phenanthroline [30]. Most important, the major degradation products of HA with Fe(III) are inorganic substances like N2, NO 2 and NO 3 [31]. Although it has been reported that adding HA into Fenton process or peroxymonosulfate activated by ferrous ion (Fe(II)/PMS) process to improve the generation of reactive oxygen species by enhancing the redox cycle of Fe(III)/Fe(II) [31,32], to the authors best knowledge, the addition of HA into PS/Fe(II) process has been never reported yet. Owing to the reduction ability from Fe(III) to Fe(II) and low rate  constants with SO 4 [16] and OH [17], HA was introduced into the PS/Fe(II) system to improve the degradation efficiency of TCE in this paper. The purposes of this study were to investigate the enhancement of TCE degradation efficiency in PS/Fe(II) system with the addition of HA and specifically focused on (1) evaluating the degradation performance of TCE in PS/Fe(II)/HA system; (2) investigating the effect of HA, Fe(II) and PS concentrations; (3) identifying the reactive oxygen species generated in PS/Fe(II)/HA system through free radical scavenging tests; (4) assessing the effects of groundwater matrix, including inorganic anions (Cl,  2 HCO 3 , SO4 , and NO3 ions) and natural organic matter (NOM), on TCE degradation performance. Through the above investigations, it is expected that HA could be effective in conversion of Fe(III) to Fe(II), therefore improve TCE degradation significantly in PS/Fe(II)/HA system. 2. Materials and methods 2.1. Materials Aqueous solutions were prepared with ultrapure water (18.2 MX cm1) produced from a Milli-Q water process (Classic DI, ELGA, Marlow, UK). Details of all chemicals used were included in the Supplementary Data (SD) Text S1. 2.2. Experimental procedures General experimental procedures were carried out in accordance with the method described in our previous study [23] by exchanging citric acid for HA. And the experimental details were

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shown in SD Text S2. The initial solution pH was unadjusted in all experiments. 2.3. Analytical methods The n-hexane extracts containing TCE were quantified by using a gas chromatograph, and the detailed parameters are shown in SD Text S3. PS concentration was determined at 400 nm through a modified spectrophotometry method with potassium iodide [33]. As ferric iron can be completely reduced to ferrous iron by excess HA, the concentration of ferrous ion (Fe(II)) and the total dissolved iron ions (Fe(II) and Fe(III)) were measured via a 1,10-phenanthroline spectrophotometry method at 512 nm [30]. The solution pH was monitored by a pH meter (Mettler-Toledo DELTA 320, Greifensee, Switzerland). Chloride ions were analyzed by an ion chromatography (Dionex ICS-I000, Sunnyvale, CA, USA). 3. Results and discussion 3.1. TCE degradation performance in Fe(II) activated PS processes by promoting the regeneration of Fe(III) to Fe(II) with different reducing agents The effects of various reducing agents (RA), i.e., hydroxylamine (HA), sodium thiosulfate (STS), ascorbic acid (AA), sodium ascorbate (SA) and sodium sulfite (SS), on accelerating the regeneration of Fe(III) to Fe(II) for PS activation to degrade TCE were investigated. The comparisons of different RA for TCE degradation as well as initial and final pH values at PS/Fe(II)/RA/TCE molar ratio of 15:2:10:1 were provided in Fig. 1a and Table S1. As can be seen, control tests (without PS) revealed a minor loss of TCE (less than 3%) during the whole experimental period and under all test conditions (data not shown). While nearly 30% of TCE was removed within 30 min in PS/Fe(II) system, indicating TCE degradation rate can be enhanced through the activation of PS by Fe(II) to generate the reactive oxygen species (sulfate radicals (SO 4 ), hydroxyl radicals (OH) and superoxide radical anions (O 2 )) (Eqs. (1)-(4)) [23]. 

 SO4 þ H2 O ! Hþ þ SO2 k < 60 M1 s1 4 þ OH

ð2Þ

2  þ S2 O2 8 þ 2H2 O ! 2SO4 þ HO2 þ 3H

ð3Þ

 2  þ    S2 O2 8 þ HO2 ! SO4 þ SO4 þ O2 þ H

ð4Þ

However, the low degradation efficiency could be interpreted with a rapid accumulation of Fe(III), the slow transformation from Fe(III) to Fe(II), and the formation of Fe(III) precipitates (i.e. iron oxide and/or hydroxide) [23], which have a lower catalytic activity and thus have a negative impact on the generation of reactive oxygen species [34]. Surprisingly, 97.9%, 69.3%, 65.5%, 71.2% and 81.1% of TCE were degraded within 30 min in PS/Fe(II) processes with the addition of HA, STS, AA, SA and SS, respectively. Those significant acceleration effects can be rationally explained by the regeneration of Fe(III) to Fe(II) with the reducing agents. For example, Zou et al. [31] reported that the addition of HA could accelerate the transformation from Fe(III) to Fe(II) during the benzoic acid degradation in Fe(II)/PMS processes via Eqs. (5)-(9); Liang et al. [22] also claimed that STS could reduce Fe(III) to Fe(II) in accordance with Eq. S1 in SD Text S4. Similarly, AA, SA and SS can also promote the regeneration of Fe(II) via Eqs. S2–S4 in SD Text S4 [35]. Among the above mentioned reducing agents, HA showed relatively higher efficiency for the destruction of TCE. This result can be explained that HA has a strong reducing property to Fe(III) [31] and a relatively lower rate  constants with SO 4 [16] and OH radicals [17]. Hence, STS, AA, SA   and SA would consume more SO 4 and OH radicals than HA during

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the volatile organic intermediate chlorinated products could be continuatively oxidized and TCE could be completely dechlorinated in PS/Fe(II)/HA system. 3.2. TCE degradation performance in PS/Fe(II)/HA system

Fig. 1. (a) Comparison of different reducing agents for TCE degradation by PS activated with Fe(II). (b) The change of chloride ion released in Fe(II)/HA activated PS system along with TCE degradation. ([TCE]0 = 0.15 mM, [PS]0 = 2.25 mM, [Fe(II)]0 = 0.3 mM, [HA]0 = [STS]0 = [AA]0 = [SA]0 = [SS]0 = 1.5 mM, T = 20 ± 0.5 °C).

TCE degradation, which are consistent with the conclusion that HA showed relatively higher range for the increase of ORP in the initial 30 seconds (Fig. S1).

NH2 OH þ Fe3þ ! NH2 O þ Fe2þ þ Hþ

ð5Þ

2NH2 O ! N2 þ 2H2 O ½Fe3þ  < ½NH2 OH

ð6Þ

Fe3þ þ NH2 O ! NHO þ Fe2þ þ Hþ

ð7Þ

2NHO ! N2 O þ H2 O

½Fe3þ  > ½NH2 OH

ð8Þ

5Fe3þ þ NH2 O þ 2H2 O ! 5Fe2þ þ NO3 þ 6Hþ ½Fe3þ  >> ½NH2 OH ð9Þ 

The change of chloride ion (Cl ) in PS/Fe(II)/HA system for TCE degradation was also analyzed and the results are shown in Fig. 1b. Theoretically, 1 mol of TCE would yield 3 mol of Cl after complete dechlorination. The results indicated that the amount of Cl released into PS/Fe(II)/HA system increased with the decreasing of TCE concentration and were completely close to the theoretical Cl concentration within 60 min. This conclusion was in agreement with no detection of any volatile organic intermediate chlorinated product within 60 min. Nevertheless, it is worth noting that some differences were existed in the reaction time, demonstrating that TCE dechlorination was incomplete and some chlorinated intermediates may yield. The difference values were gradually narrowing over time, and finally can be negligible in 60 min, indicating that

3.2.1. Effect of HA concentration on TCE degradation in PS/Fe(II)/HA system In order to investigate the role of HA, the effect of HA concentration on TCE degradation in PS/Fe(II)/HA system was analyzed, and the results are shown in Fig. 2a and Table S2. As it can be seen, the increase of HA concentration from 0 to 1.5 mM led to an enhancement in TCE degradation efficiency from 28.9% to 97.9% within 30 min (Table S2). While the further increase of HA concentration resulted in a decrease in TCE removal. It should be noted that as soon as PS was added, the solution pH dropped to 2.0–3.0 immediately (Table S2), and HA mainly existed in the form of NH3OH+ with pKa1 = 5.96 [31]. Although increasing NH3OH+ concentration can promote the transformation of Fe(III) to Fe(II), the  reactive oxygen species (SO 4 and OH) generated in PS/Fe(II)/HA system could be swept by excess NH3OH+ with high reaction rate, i.e., k < 5.0  108 M1 s1 and k = 1.5  107 M1 s1 for OH [17] and SO 4 [18], respectively. What is more, with the increase of HA concentration, more Cl would be introduced into system along with HA addition, which had a significant scavenging function to   SO 4 and OH [23]. To better understand the role of HA in PS/Fe(II)/HA system during TCE degradation, the decomposition of PS, and the variations of Fe(II) and the total dissolved iron ions (Fe(II) and Fe(III)) ratio were also measured and the results are shown in Fig. 2b and c. The PS results shown in Fig. 2b indicated that PS decomposition was dependent on the concentration of HA, the higher HA concentration used resulted in the higher PS decomposition. It is noteworthy that, with the addition of HA, the total dissolved iron ions concentrations were changeless and equal to the dosage concentration of Fe(II) through determination (data not shown). As shown in Fig. 2c, with the increasing initial HA concentration, the conversion from Fe(II) to Fe(III) was slowed down at a fixed PS/Fe(II)/TCE molar ratio of 15:2:1. Apparently with the increase of HA concentration to a certain degree (i.e., 3 mM and 7.5 mM), Fe(III) began to reduce to Fe(II) partly, and the higher of HA concentration, the reduction time was earlier. Therefore, in order to enhance TCE degradation at the highest extent and reduce the cost, a proper concentration of HA should be selected in practical application, and 1.5 mM was the optimal HA concentration in this process. 3.2.2. Effect of Fe(II) concentration on TCE degradation in PS/Fe(II)/HA system In PS/Fe(II)/HA system, Fe(II) was used as the transition metal for the activation of PS to generate the reactive oxygen species. Thus, the influence of Fe(II) concentration at the range of 0–3 mM on TCE degradation was investigated, and the results are displayed in Fig. 3a and Table S2. The result demonstrated that TCE removal efficiency was significantly increased with the addition of Fe(II). Concretely, TCE degradation increased with the increase of Fe(II) concentration from 0 to 0.3 mM; however, further increases of Fe(II) concentration resulted in a decrease of TCE removal, especially in initial 30 seconds (Table S2). This phenomenon was logical with the following reasons: increasing Fe(II) concentration can generate more reactive oxygen species (Eqs. (1)-(4)) and therefore increase TCE degradation; however, excess Fe(II) can also act as an effective scavenger of SO 4 at its high concentration as expressed by Eq. (10) [20], resulting in a decrease of TCE degradation. Therefore, it could be concluded that an essential concentration of Fe(II) was required to effectively activate PS to generate a sufficient

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less influence on the PS decomposition due to the presence of HA, while further increasing Fe(II) concentration resulted in an increase of PS decomposition because of the activation of PS by excess Fe(II). The results further suggest that an increase of Fe(II) concentration into PS/HA process significantly accelerated the decomposition of PS, hence a suitable dosage of Fe(II) could improve the degradation efficiency of TCE.

3.2.3. Effect of PS concentration on TCE degradation in PS/Fe(II)/HA system PS concentration was a critical parameter as the source of reactive oxygen species in PS/Fe(II)/HA system, thus the influence of PS

Fig. 2. The effect of HA concentration on (a) TCE degradation, (b) the decomposition of PS, and (c) the variations of Fe(II) to total iron ions ratio in PS/Fe(II)/HA process. ([TCE]0 = 0.15 mM, [PS]0 = 2.25 mM, [Fe(II)]0 = 0.3 mM, T = 20 ± 0.5 °C).

amount of reactive oxygen species, but an excess concentration of Fe(II) would be detrimental to TCE degradation. 

SO4 þ Fe2þ ! Fe3þ þ SO2 4

k ¼ 4:6  109 M1 s1

ð10Þ

The variations of Fe(II) and the total dissolved iron ions ratio, and the decomposition of PS were also monitored during the reaction with different initial concentration of Fe(II) ranging from 0.03 to 3 mM. As shown in Fig. 3b, Fe(II) was consumed rapidly and the ratio of Fe(II) to total iron ions decreased sharply once the reaction was started. Then Fe(II) concentration kept relatively constant in different concentration of Fe(II) for the existence of residual HA. At the same time, the results shown in Fig. 3c demonstrated that an increase in Fe(II) concentration at the range of 0–0.3 mM had

Fig. 3. The effect of Fe(II) concentration on (a) TCE degradation, (b) the variations of Fe(II) to total iron ions ratio, and (c) the decomposition of PS in PS/Fe(II)/HA process. ([TCE]0 = 0.15 mM, [PS]0 = 2.25 mM, [HA]0 = 1.5 mM, T = 20 ± 0.5 °C).

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concentration on TCE degradation was also evaluated, and the results are shown in Fig. 4a and Table S2. As it can be seen, about 41% of TCE removal efficiency was increased with the addition of 0.3 mM PS within 30 min. With the increasing of PS dosage over the tested range, TCE degradation was increased. Hence, the presence of sufficient persulfate was necessary because PS was the source of the reactive oxygen species responsible for TCE degradation, while it is desirable to limit the excess PS concentration used for economic reason. Based on the aforementioned data and analysis, it could be inferred that a minimum PS concentration of 2.25 mM was necessary when a Fe(II)/HA/TCE molar ratio of 2/10/1 was used.

Meanwhile, the PS decomposition (Fig. 4b) and the variations of Fe(II) to total iron ions ratio (Fig. 4c) were also measured at the same conditions. As for PS/Fe(II)/HA/TCE molar ratios of 2:2:10:1, 5:2:10:1 and 10:2:10:1 shown in Fig. 4b, persulfate was depleted within 30 min due to the insufficient PS concentration and the excess residual HA, which were equal to or much less than the stoichiometric amount of PS needed for the transformation of ferrous ion according to Eqs. (1) and (5). It should also be noted that Fe(II) concentration began to rise around or after 5 min with PS/Fe(II)/ HA/TCE molar ratios of 2:2:10:1 and 5:2:10:1 because of the complete consumption of PS and the excess residual HA. As the PS/Fe(II)/HA/TCE molar ratios increased, the percentages of residual PS was risen and Fe(II) concentration were approximately undetected within 30 min, suggesting that the recovery of Fe(II) and TCE degradation were strongly affected by the addition of PS in PS/Fe(II)/HA systems. 3.3. Scavenging tests for the reactive oxygen species identification in PS/Fe(II)/HA system It has been reported that a suite of reactive oxygen species, i.e.,   SO 4 , OH and O2 , were often generated in persulfate activated by citric acid chelated ferrous ion system [23] and may also be generated in PS/Fe(II)/HA system. Owing to the high rate constants with  7 1 1 SO s ) [36] and OH (k = 1.9  109 M1 s1) 4 (k = 8.2  10 M [17], isopropanol (IPA) is an effective scavenger for both SO 4 and  OH. Because the rate constant between tert-butyl alcohol (TBA) with OH (k = 5.2  108 M1 s1) [17] is approximately 1000 times 5 1 1 greater than that with SO s ) [36], TBA is an 4 (k = 8.4  10 M  effective scavenger for only OH but not for SO 4 . Moreover, 1,4benzoquinone (BQ) is an effective scavenger of O 2 through rapid electron transfer to generate benzoquinone radicals [37]. Based on these properties of the scavengers, to determine whether these free radicals were generated in PS/Fe(II)/HA system, the radical scavenger tests were carried out by repeating the reaction of TCE with the addition of radical scavengers IPA, TBA or BQ, respectively.  Furthermore, to investigate other free radicals beside SO 4 , OH and  O2 generated in PS/Fe(II)/HA system, the effect of the combined IPA and BQ was also evaluated. As shown in Fig. 5, when only OH was scavenged with the excess TBA (0.1 M), the removal of TCE was decreased from 97.9% to 38.9% within 30 min, suggesting that approximately 59.0% of the TCE oxidation in PS/Fe(II)/HA system was due to the activity of OH. Nevertheless, the addition of excess IPA (0.1 M) decreased the degradation efficiency of TCE by approximately 

Fig. 4. The effect of PS concentration on (a) TCE degradation, (b) the decomposition of PS, and (c) the variations of Fe(II) to total iron ions ratio in PS/Fe(II)/HA process. ([TCE]0 = 0.15 mM, [Fe(II)]0 = 0.3 mM, [HA]0 = 1.5 mM, T = 20 ± 0.5 °C).

Fig. 5. The effect of scavengers on TCE degradation performance in PS activated by Fe(II)/HA system. ([TCE]0 = 0.15 mM, [TBA]0 = 0.1 M, [IPA]0 = 0.1 M, [BQ]0 = 10 mM, [PS]0 = 2.25 mM, [Fe(II)]0 = 0.3 mM, [HA]0 = 1.5 mM, T = 20 ± 0.5 °C).

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84.9% compared to free radical unscavenged system within 30 min, while the small amount of TCE degradation that could not be eliminated was probably attributed to the generation of O 2 in PS/Fe(II)/ HA system. When comparing the difference of TCE degradation efficiency between the addition of OH scavenger TBA and the com   bined SO 4 and OH scavenger IPA, it is obvious that SO4 also contributed about 25.9% to TCE degradation. Furthermore, with the addition of excess IPA (0.1 M) and BQ (10 mM) which can sweep  all the aforementioned free radicals of OH and SO 4 , O2 , the degradation efficiency of TCE was completely inhibited, demonstrating   that no other free radicals except SO 4 , OH and O2 were generated  in PS/Fe(II)/HA system, and O2 contributed 10.4% to TCE removal. Moreover, TCE degradation was also inhibited in the presence of excess BQ (10 mM) within 30 min, further demonstrated the generation of O 2 in PS/Fe(II)/HA system. According to the inhibition effect of TBA, IPA and BQ on   TCE degradation, it could be concluded that SO 4 , OH and O2 were generated and OH was the predominant one in PS/Fe(II)/HA process. 3.4. Effect of solution matrix on TCE degradation performance in PS/ Fe(II)/HA system  2 Inorganic anions such as Cl, HCO 3 , SO4 , and NO3 ions are ubiquitous in contaminated groundwater, necessitating a study of the influence of these ions at different levels on TCE degradation performance in PS/Fe(II)/HA system. It can obviously be found from Fig. 6 and Table S3 that the inorganic anions over the range of concentration investigated had negative effects on TCE degradation.

As shown in Fig. 6a, the effect of Cl ions on TCE removal was not obvious at low concentration of 1.0 mM, while with the increasing concentration of Cl ion, marked inhibition occurred. A possible explanation for this phenomenon is that Cl performed  as radical scavengers of SO 4 and OH, resulting in competition with these reactive oxygen species for reaction with TCE, and therefore inhibited TCE degradation, according to the following related reaction Eqs. S5-S12 in SD Text S5 [38–40]. Compared with Cl ion, a much more significant inhibitive effect was observed for HCO 3 ions, and extensive inhibition occurred with higher concentrations. The significantly suppressive effect of HCO 3 ion on TCE removal can be rationally explained by the following reasons: the presence of HCO 3 ion had a significant   scavenging effect on SO 4 , OH and O2 radicals in the aqueous phase as shown in Eqs. S13–S17 in SD Text S6 [41–43]; Moreover, the initial solution pH elevated as the concentration of   HCO 3 ion increased (Table S3), and thus the generation of SO4 and OH were suppressed under basic condition [44,45], leading to a decrease of TCE degradation efficiency. With the addition of SO2 4 ion, a slightly inhibiting effect was observed at the tested anion concentration ranges on TCE degradation in PS/Fe(II)/HA system (Fig. 6c). Assuming that the degradation of TCE involving the depletion of SO 4 radical can be simplified into Eq. (11), and then the corresponding Nernst equation of Eq. (11) can be formulated as Eq. (12) (Logic). 

SO4 þ e ! SO2 4

ð11Þ


RT ½ SO4  E  SO4 ¼ Ehð SO =SO2 Þ þ ln 4 4 zF ½SO2 

ð12Þ

4

2 Fig. 6. The effect of inorganic anions on TCE degradation performance in Fe(II)/HA activated PS system. (a) Cl ion, (b) HCO ion, and (d) NO 3 ion, (c) SO4 3 ion. ([TCE]0 = 0.15 mM, [PS]0 = 2.25 mM, [Fe(II)]0 = 0.3 mM, [HA]0 = 1.5 mM, T = 20 ± 0.5 °C).

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In conclusion, for practical use of PS oxidant applied to in situ remediation application of contaminated sites containing chlorinated solvent, the background values of inorganic anions and NOM existed in groundwater/soil should be considered and measured in determination of the PS dosage. 4. Conclusion

Fig. 7. The effect of humic acid on TCE degradation performance in PS activated by Fe(II)/HA system. ([TCE]0 = 0.15 mM, [PS]0 = 2.25 mM, [Fe(II)]0 = 0.3 mM, [HA]0 = 1.5 mM, T = 20 ± 0.5 °C).

where Eð SO =SO2 Þ represents the half-reaction reduction potential, 4 4 Ehð SO =SO2 Þ stands for the standard half-reaction reduction potential, 4

4

R is the universal gas constant of 8.3145 J K1 mol1, T is the absolute temperature of 293.15 K in this study, z is the number of electrons transferred in the half-reaction, and F is the Faraday constant of 9.63845  104 C mol1. Based on the Eq. (12), it is observed 2 that the oxidation reduction potential of SO can be influ4 /SO4 2 enced by the concentration of SO4 ion. Hypothesizing that a complete generation of SO 4 radical could be achieved through Eq. (1) in the first-stage,, it can be calculated that Eð SO =SO2 Þ would 4

4

be reduced by 0.05823 V and 0.3408 V, respectively, with the ion from 1 to 10 and 100 mM increasing concentration of SO2 4 under tested conditions. Hence, the higher SO2 concentration in 4 the solutions, the lower potential of Eð SO =SO2 Þ was achieved, which 4

In this study, through the investigation of TCE degradation with different reducing agents in Fe(II) activated PS process, it was found that hydroxylamine (HA) was most efficient in TCE degradation due to its strong reducing property in transformation of Fe(III) into Fe(II). TCE could be completely degraded within 30 min and the Cl release test confirmed that TCE could be completely dechlorinated in PS/Fe(II)/HA system. The optimum molar ratio of PS/Fe(II)/HA/TCE was indentified to be 15:2:10:1, and further increase of HA and Fe(II) concentrations resulted in a decrease in TCE removal. TCE degradation was improved with the increasing of PS concentration. In addition, the radical scavenging experiments clearly confirmed that the primary reactive oxygen species     2 were SO 4 , OH and O2 in PS/Fe(II)/HA system. Cl , HCO3 , SO4  and NO3 anions had inhibitory effects on TCE removal, and the suppressive effects of inorganic anions could be ranked in an   2 ascending order of NO 3 < SO4 < Cl < HCO3 . NOM had a similar effect as the anions in inhibition of TCE degradation within the initial 30 s, whereas overall TCE degradation was still achieved over 30 min. Briefly, Fe(II)/HA activated PS process is a highly promising technique for remediation of the contaminated sites containing TCE, but more complex constituents existed in groundwater should be carefully considered for its widespread practical application.

4

directly inhibit the degradation performance of TCE. The same inhibition effect on TCE degradation was observed with the dosage of NO 3 ion and the results are shown in Fig. 6d and Table S3. One possible explanation is that with the increasing of NO 3 concentration, an autocatalytic process shown in Eq. (13) could occur with the presence of sufficiently concentrated NO 3 ion [46], leading to a consumption of HA and an insufficient regeneration of Fe(II). Based on the aforementioned data as shown in Fig. 6 and Table S3, it could be concluded that these inorganic anions could inhibit the degradation efficiency of TCE and their suppressive effects can be ranked in an ascending order of   2 NO 3 < SO4 < Cl < HCO3 with the same molar concentration in PS/Fe(II)/HA system.

Acknowledgements This study was financially supported by the grant from the National Environmental Protection Public Welfare Science and Technology Research Program of China (No. 201109013), the National Natural Science Foundation of China (Nos. 41373094 and 51208199), the Shanghai Natural Science Funds (No. 12ZR1408000), China Postdoctoral Science Foundation (No. 2013T60429), and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2015.04. 031. References

NO3



þ NH2 OH ! NO þ

NO2

þH

þ

ð13Þ

Generally, humic acid is a major fraction of natural organic matter (NOM) and ubiquitous in groundwater and soil environments, so its effect on TCE degradation performance in PS/Fe(II)/HA process was investigated and the results are shown in Fig. 7 and Table S3. As it can be seen, the degradation efficiency of TCE at first 30 s decreased from 49.9% to 48.3%, 46.1% and 38.0% when humic acid concentration increased from 0 to 1.0, 10.0 and 100 mg/L, respectively. However, overall TCE degradation was still achieved over 30 min at all of the humic acid dosages. A possible explanation for this phenomenon is that humic acid may serve as a free  radical scavenger through competing with SO 4 and OH and a potential contributor to oxidant consumption [23], while the inhibitory effect can be neglected after 30 min due to the existing of residual PS.

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